Precise Level Indication

  • January 29, 2009
  • News
There are various methods to precisely measure the level in a container. In environments that allow for direct contact, devices based on conductivity such as the Baumer LSP Transmitter are most suitable. They represent an economic alternative to much more complex measuring systems and are suited most of all for continuous level measuring in the food industry.Measuring and monitoring of levels in different areas requires a reliable measuring system at reasonable costs. Level switches can only measure the tank content in selective places of the tank and are, thus, insufficient to precisely measure the tank contents regardless of the level. Such an accurate measurement is necessary in cases when intermixtures or dosing of liquids are to be carried out, when a fine control is required or when consumption and waste levels need to be controlled. Baumer offers a range of economic measuring systems, which are based on different technologies. Devices for hydrostatic pressure and differential pressure measurement are based on Piezo-resistive, ceramics, capacity or thin-film technologies. Levels can also be measured with ultrasonic and optical sensors as well as with conductivity based devices. This means both contact and non-contact measurement requirements are welcome because Baumer can offer its customers optimal application-tailored solutions with suitable technology. This extensive range of level sensors is especially suited for measurements in industrial applications, in chemical and fine chemical engineering as well as the food and beverage industry. The Measuring Principle for Continuous Level MeasuringAmong conductivity based sensors, the level measuring device LSP 05x stands out because it is reliable, easy to install and conducts highly accurate measurements. The measuring method is based on a low resistive measuring rod which dips into a conductive liquid. An oscillator generates an alternating current, which is fed to the whole measuring rod. Depending on the respective level, a proportional current flow is captured between the rod and the metallic tank wall or a reference electrode and then routed to an amplifier. The liquid’s resistance decreases proportionally to the immersed part of the rod. If the liquid is homogeneous, an absolute linear measurement of the fill level and thus a linear signal output of 4 … 20 mA is provided.The rod in direct contact with the media is made of stainless steel 1.4404 and a highly resistant media-sided plastic seal made out of PEEK. Depending on the version, level can be measured in a wide variety of situations including pipes, small containers and tanks up to 3 metres deep containing all liquids, including those which are partly aggressive, tough or even pasty such as ketchup, honey or tooth-paste. The only precondition for this measuring principle is that the liquid has a minimum conductivity of 1S/cm. The fact that it is not affected by pressure, density, temperature or DK value opens up a wide application range for this technology.The sensor can easily be programmed using two push buttons on the device. The simple Teach-in procedure allows setting levels from 50 mm to up to the whole length of the rod excluding the 5 mm at the end of the measuring rod, which is not active. This also applies if the device is mounted at the bottom of the tank. In this case, the output signal can be reversed in the “Teach-in” process. On the media side a maximum pressure of 16 bar and a permanent temperature from -20 to up to 140°C are possible. Even short term temperature peaks of 150°C are possible without the device getting damaged. Therefore, the sensor is very well suited for use in CIP and SIP facilities. The excellent metering accuracy of ±0.5 per cent of the respective rod length and the short response time of Tm=10 ms enable a precise level control. A stabilised voltage supply of 18 ... 36 VDC at 200 mA guarantees secure operation.Successful Applications in Critical EnvironmentsDue to the hygienic installation conditions, the robust, easy to clean stainless steal casing body of the protection class IP 67 including screw cap with recessed grips and O-ring seal, the stainless steal made measuring rod as well as the 3A-suitable process connection, the device can be used in the food and beverage industry as well as in pharmaceutical and biochemistry. Compared to other customary measuring principles, this method provides a cost effective solution for the monitoring of small and medium-sized containers and tanks. The inherent design and working principle make this sensor immune to the effects of both foam and product adhesion ensuring dependable accuracy at all times. Teflon coating of areas outside the measurement zone ensures they are not subject to false signals. Should constructural conditions not allow a straight mounting of the measuring rod, it can also be angled and installed with a bending radius from ≥ 10 cm to up to 90°. This is for example required if the device can only be installed from the side.Different versions cover applications where there are high ambient temperatures at the measuring point, necessary cleaning with strong jets of water or high pressure cleaners. The measuring rod is mounted with an empty housing at the measuring point. It is connected over a cable, which can be up to five metres long, with the amplifier in an external housing that can be mounted in a suitable place. These features and the different ways to install the device make it a highly flexible solution and offer advantages to most industries.Examples of UseThe production of chocolate mousse or measuring in tanks with moving liquids poses a specific challenge to level measuring devices. The following examples show cases in which workable and economic solutions were found with LSP 05x. In one case, a horizontal vessel meant the measuring rod could only be installed sideways. The media was chocolate mousse, and the level had to be kept constant to ensure the downstream dosing system was always provided with an even amount. The rod was angled 45° into the media and the sensor horizontal surface was coated with Teflon to prevent false signals from foam. The angled side piece remained uncoated to ensure a signal path which served to record the level.In another case, the level of a moving liquid in a concrete basin had to be very precisely measured. The LSP measuring rod was centrally inserted in a stainless steal pipe with an inner diameter of 30 mm. The performance check showed a correct measurement.These solutions for accurate level measuring have been worked out in cooperation with customers and system partners and show clearly that the conductivity based measuring system can be employed successfully even under adverse conditions.ConclusionsThe practical examples show that reliability and robustness on one side and highly accurate measurements on the other side do not have to be mutually exclusive. In small to medium-sized tanks or in pipes allowing for direct contact measurements, the LSP 05x with its outstanding features represents an economic alternative to other expensive measuring systems. These generally require a time-consuming and complex installation and commissioning procedure. Providing high protection class, completely stainless steel construction, adjustable measuring range beginning with 50 mm as well as the high accuracy of ±0,5 per cent of the chosen active rod length, the LSP 05x level measuring device is very versatile. Learn More

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